According to the YouTube account of Mauricio Ruiz, the following footage was shot from the window of a SouthWest Airlines flight from La Guardia airport to Houston, Texas.

Ruiz goes on to say that he has submitted the video to FAA officials and is waiting further review from them.

The video shows an object outside the cabin of the airplane. The object does not appear to be a reflection of anything from inside. It does not look like a reflection of light on the window, especially not when the camera zooms in and we can see details on the supposed craft.

There's something not right about it. A few times when he zoomed out, it completely disappeared. Also, he wouldn't be able to get that clear of details when zooming in like that. It should be a fuzzy mess, but is magically clear somehow.

Also, wouldn't the TCAS on the plane detect if an object was close to it?
I would imagine the TCAS would be enabled on an airliner flying over populated areas at night, and surely they wouldn't just ignore an object flying next to it.